xfs_io: Add cachestat syscall support

xfs_io: Add cachestat syscall support

This adds -c "cachestat off len" command which uses cachestat() syscall
[1]. This can provide following pagecache detail for a file.

- no. of cached pages,
- no. of dirty pages,
- no. of pages marked for writeback,
- no. of evicted pages,
- no. of recently evicted pages

[1]: https://lore.kernel.org/all/20230503013608.2431726-3-nphamcs@gmail.com/T/#u

Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Reviewed-by: Andrey Albershteyn <aalbersh@kernel.org>
[aalbersh remove [] from command arguments help]